It has been quite warm, but I think inexperience has alot to do with it. I think it is likely to increase as the 'general public' have a tendancy to start running a few miles one week and then start entering half marathons instead of building up the distances slowly.
I felt well rough after the Portland 10 one year - pins and needles in the head, finishing funnel wobbling about etc etc. Lesson leant? Never have 'a few pints' the night before!